Various processes and catalysts exist for the homopolymerization or copolymerization of olefins, such as propylene. For many applications, it is desirable for a polyolefin to have a high Mw while having a relatively narrow Mw/Mn. A high Mw, when accompanied by a narrow Mw/Mn, provides a polyolefin with high strength properties.
Traditional Ziegler-Natta catalysts systems comprise a transition metal compound co-catalyzed by an aluminum alkyl and are typically capable of producing polyolefins having a high Mw, but with a broad Mw/Mn.
More recently metallocene catalyst systems have been developed wherein the transition metal compound has one or more cyclopentadienyl, indenyl or fluorenyl ring ligands (typically two). Metallocene catalyst systems, when activated with cocatalysts, such as alumoxane, are effective to polymerize monomers to polyolefins having not only a high Mw but also a narrow Mw/Mn.
Particular focus has been directed to metallocenes containing substituted, bridged indenyl rings, since these materials are particularly effective in producing isotactic propylene polymers having high isotacticity and narrow Mw/Mn. Considerable effort has been made toward obtaining metallocene produced propylene polymers having ever-higher molecular weight and melting point, while maintaining suitable catalyst activity. Researchers currently believe that there is a direct relationship between the way in which a metallocene is substituted, and the molecular structure of the resulting polymer. For the substituted, bridged indenyl type metallocenes, it is believed that the type and arrangement of substituents on the indenyl groups, as well as the type of bridge connecting the indenyl groups, determines such polymer attributes as molecular weight and melting point. Unfortunately, it is impossible at this time to accurately correlate specific substitution patterns with specific polymer attributes, though minor trends may be identified, from time to time.
For example, U.S. Pat. No. 5,840,644 describes certain metallocenes containing aryl-substituted indenyl derivatives as ligands, which are said to provide propylene polymers having high isotacticity, narrow molecular weight distribution and very high molecular weight. Likewise, U.S. Pat. No. 5,936,053 describes certain metallocene compounds said to be useful for producing high molecular weight propylene polymers. These metallocenes have a specific hydrocarbon substituent at the 2 position and an unsubstituted aryl substituent at the 4 position, on each indenyl group of the metallocene compound. Neither U.S. Pat. No. 5,840,644 nor 5,936,053 disclose carbazole substituents.
Also known in the art are unbridged indenyl based metallocenes having bulky substituents on the indenyl ligand, thus providing “fluxionality” to the activated catalyst. Metallocenes of this type are believed to produce “elastomeric” polypropylene. Science, 1995, 267, 217 discusses such catalysts.
U.S. Pat. No. 6,458,982 discloses compositions containing aromatic heterocyclic substituents on cyclopentadienyl or indenyl based metallocenes (non-bridged), but that are not bonded to the cyclopentadienyl or indenyl ring via the heteroatom. U.S. Pat. No. 6,169,051 B1 discloses compositions containing aromatic heterocyclic substituents on cyclopentadienyl or indenyl based metallocenes (bridged), but that are not bonded to the cyclopentadienyl or indenyl ring via the heteroatom.
References containing non-aromatic heterocyclic substituents on cyclopentadienyl or indenyl based metallocenes (bridged), and that are bonded to the cyclopentadienyl or indenyl ring via the heteroatom include: J. Organometallic Chem. 1996, 519, 269; Organometallics 2000, 19, 1262; U.S. Pat. No. 5,756,608; U.S. Pat. No. 5,585,509; and European Patent No. 0 670 325 B1.
U.S. Pat. No. 6,479,646 discloses bridged bis-indenyl metallocene compounds having cyclic heteroatom-containing substituents, but in all the compounds exemplified the substituents are bonded to the indenyl ligand via the 2-position of the cyclic heteroatom group.
U.S. Pat. No. 7,276,567 discloses bridged and unbridged indenyl metallocene compounds having cyclic heteroatom-containing substituents bonded to the indenyl ligand through a nitrogen or a phosphorus ring heteroatom.
JP3323347B2 discloses bridged bis-indenyl metallocenes in which the indenyl ligands are substituted at the 4-position by pyrrol-1-yl substituents including ring-fused hydrocarbyl substituted pyrrol-1-yl substituents. When combined with a cocatalyst, the metallocenes are said to effective in producing high molecular weight α-olefin polymers over a temperature range of −78° C. to 200° C., preferably −20° C. to 100° C.
US 2002/0002261 discloses bis[2-(2-furyl)indenyl]zirconium dichloride and or bis[2-(2-furyl)-4-phenylindenyl]zirconium dichloride for use in making polypropylene. Likewise U.S. 2001/0031834 discloses polypropylene compositions made using indenyl metallocenes, which can be substituted with hetero-aromatic groups, preferably a 2-furyl group, a 3-furyl group, a 2-thienyl group, a 3-thienyl group, a 2-pyrrolyl group, a 3-pyrrolyl group, a 2-pyridyl group, a 3-pyridyl group, a 4-pyridyl group, a benzofuryl group, a benzothienyl group, a 3-indolyl group, a 1-quinolyl group, or a 3-quinolyl group. US 2001/0053833 discloses compounds that are bridged indenes that have heterocyclic substituents on the indene with the proviso that at least one of the heterocyclic substituents has another substituent on it. Examples are all with 2-substituted heterocycles including bis(2-(2-(5-methyl)-furyl)-4,5-benzoindenyl)zirconium dichloride, bis(2-(2-(5-phenyl)-furyl)-indenyl)zirconium dichloride, rac-dimethylsilylene bis(2-(2-(5-methyl)furyl)-4-phenylindenyl)zirconium dichloride, and rac-dimethylsilylene bis(2-(2-(5-methyl)furyl)-indenyl)zirconium dichloride.
WO 2006/097497 discloses bridged bis-indenyl metallocene compounds, in which each indenyl ligand is substituted at its 2-position with a C1-C40 hydrocarbon radical and at its 4 position with an aromatic 5 or 6 membered ring.
It is also desirable to produce polypropylene in a homogeneous process such as a solution or a supercritical process. High polymerization temperature is useful for the homogeneous process to maintain polypropylene produced in solution and not phase separate from the polymerization medium. However, for almost all catalysts, polypropylene produced at higher temperatures has lower molecular weight and lower melting temperature, and hence poorer end-use properties. It is challenging to producing polypropylene with high molecular weight and high melting temperature in a homogeneous process.
According to the present invention, it has now been found certain N-bonded carbazol-9-yl substituted bridged bis-indenyl metallocene compounds, when combined with a suitable cocatalyst, are effective under solution (including supercritical) polymerization conditions to produce olefin polymers with exceptionally high molecular weight. Surprisingly, propylene polymers produced using these N-bonded carbazol-9-yl substituted metallocene compounds exhibit significantly higher molecular weight than similar polymers produced using the equivalent N-bonded pyrrol-1-yl and indol-1-yl substituted metallocenes. Also surprising is the higher polymer melting point for propylene polymers produced using these carbazol-9-yl substituted metallocene compounds relative to those produced using the equivalent pyrrol-1-yl and indol-1-yl substituted metallocenes.